Heat-sensitive recording medium

ABSTRACT

A polyurethane resin is composed of a polyol component, a polyisocyanate component and optionally, a chain extender component. At least a portion of the polyol component or chain extender component is a siloxane polyol component represented by the following formula (1) and/or formula (2): ##STR1## wherein R means an alkyl group, R&#39; denotes a hydrogen atom or C 1-5  alkyl group, k, l and m stand for 1-250, 0-5 and 0-50 respectively, and n is an integer of 1-3 in the formula (1) and an integer of 2-3 in the formula (2). A thermal recording material containing the polyurethane resin as a heat-resistant layer is also disclosed.

This is a division of application Ser. No. 07/280,583, filed on Dec. 6,1988, now U.S. Pat. No. 4,942,212.

BACKGROUND OF THE INVENTION

1) Field of the Invention

This invention relates to polyurethane resins and heat-sensitiverecording media (hereinafter called "thermal recording materials"), andmore specifically to silicone-modified polyurethane resins excellent instick-free property, antiblocking property and flexibility and also tothermal recording materials making use of such resins, now U.S. Pat. No.4,942,212.

2) Description of the Related Art

Polyurethane resins have already found a wide variety of utility asbinders for various coating agents, coating compositions and inks,films, formed and molded articles, etc. Polyurethane resins suitable forvarious applications have been proposed.

These polyurethane resins can be obtained basically by reacting a polyolcomponent, a polyisocyanate component and if necessary, a chainextender. Polyurethane resins of various physical properties have beenprovided depending on the kinds of these individual components, theircombinations and the like.

For some applications of polyurethane resins, for example,heat-resistant layers of thermal recording materials and magneticrecording media, stick-free property to thermal heads, antiblockingproperty between films themselves, etc. are however required in someinstances. In general, sufficient flexibility is also required for theseapplications in order to permit the formation of thin films.

Introduction of one or more aromatic components and many polar bondsinto a polyurethane chain is however indispensable to impart hightack-free property and the like. In many instances, this has led tosacrifice of flexibility.

There is accordingly an outstanding demand for the development of apolyurethane resin excellent in high-temperature tack-free property,antiblocking property and the like and superb in flexibility.

SUMMARY OF THE INVENTION

The present inventors have carried out an extensive investigation with aview toward solving the above-mentioned drawbacks of the conventionaltechniques and hence meeting the aforementioned demand. As a result, ithas been found that this objective can be achieved by forming apolyurethane resin with a specific polyurethane component.

In one aspect of this invention, there is thus provided a polyurethaneresin composed of a polyol component, a polyisocyanate component andoptionally, a chain extender component. At least a portion of the polyolcomponent or chain extender component is a siloxane polyol componentrepresented by the following formula (1) and/or formula (2): ##STR2##wherein R means an alkyl group, R' denotes a hydrogen atom or C₁₋ 5alkyl group, k, l and m stand for 1-250, 0-5 and 0-50 respectively, andn is an integer of 1-3 in the formula (1) and an integer of 2-3 in theformula (2).

In another aspect of this invention, there is also provided a thermalrecording material containing the polyurethane resin as a heat-resistantlayer.

Bonding of polysiloxane bonds as pendants to the backbone of apolyurethane resin has made it possible to provide a polyurethane resinexcellent in tack-free property, antiblocking property and the likewhile retaining the excellent flexibility of the former polyurethaneresin.

The polyurethane resin according to this invention has superb solubilityin various solvents and can provide film excellent not only inflexibility but also in high-temperature tack-free property,anti-blocking property and the like, so that it is useful for variousapplications, for example, as a binder for the formation of magneticlayers of magnetic recording media such as magnetic tapes, aback-coating material applied to the back sides of such magneticrecording media, a binder for transfer films, a binder for the formationof heat-resistant layers effective for the prevention of sticking of athermal head, a base material for synthetic leather, a fiber-coatingagent, a surface treatment agent, a binder for the formation of releaselayers such as parting paper, a binder for coating compositions andprinting inks, etc.

Use of the polyurethane resin according to this invention, whichcontains residual groups of the specific siloxane compound, for theformation of a heat-resistant layer for a thermal recording materialrenders the thermal recording material free from the sticking problem ofa thermal head thereto.

DESCRIPTION OF PREFERRED EMBODIMENTS

This invention will hereinafter be described in more detail based oncertain preferred embodiments of this invention.

The polyurethane resin containing as pendants residual groups of thesiloxane compound of the above formula (1) and/or that of the aboveformula (2) can be obtained by using the siloxane compound of the aboveformula (1) and/or that of the above formula (2) as the whole portion ora portion of a polyol or chain extender when the polyurethane resin isobtained by reacting the polyol, a polyisocyanate and if necessary, thechain extender, etc.

The siloxane compounds of the formulae (1) and (2) are sold commerciallyand readily available on the market these days. These commercialsiloxane compounds are all useful in the practice of this invention.Also usable in this invention may include intermediates obtained byreacting one of such siloxane compounds and a polyisocyanate, which willbe described subsequently, in such a way that at least either reactivegroups of the siloxane compound or isocyanate groups of thepolyisocyanate are left over, for example, an intermediate obtained byreacting a bifunctional siloxane compound and a polyfunctionalpolyisocyanate with isocyanate groups richer or in contrast, anintermediate obtained by reacting them with the reactive group of thesiloxane compound richer.

Polyols, which have heretofore been known to be useful for theproduction of polyurethanes, can all be employed as polyols usable incombination with the siloxane compounds of the formula (1) and/or thoseof the formula (2). For example, preferred are those containing hydroxylgroups as terminal groups and having a molecular weight in a range of300-4,000, including:

Polyethylene adipate,

Polyethylenepropylene adipate,

Polyethylenebutylene adipate,

Polydiethylene adipate,

Polybutylene adipate,

polyethylene succinate,

Polybutylene succinate,

Polyethylene sebacate,

Polybutylene sebacate,

Polytetramethyleneether glycol,

Poly-ε-caprolactonediol,

Polyhexamethylene adipate, and

Polypropylene glycol; and those containing a suitable proportion ofpolyoxyethylene chains in the above polyols.

Organopolyisocyanates which have been known to date can all be used. Thefollowing organopolyisocyanates may be mentioned by way of example aspreferred ones:

4,4'-Diphenylmethane diisocyanate (MDI),

Hydrogenated MDI,

Isophorone diisocyanate,

1,3-Xylylene diisocyanate,

1,4-Xylylene diisocyanate,

2,4-Tolylene diisocyanate,

2,6-Tolylene diisocyanate,

1,5-Naphthalene diisocyanate,

m-Phenylene diisocyanate, and

p-Phenylene diisocyanate.

It is of course possible to use urethane prepolymers and the like, whichare each obtained by reacting one of these organopolyisocyanate with apolyol or polyamine of a low molecular weight.

Chain extenders which have been known conventionally can all be used.Preferred examples may include:

Ethylene glycol,

propylene glycol,

Diethylene glycol,

1,4-Butanediol,

1,6-Hexanediol,

Ethylenediamine,

1,2-Propylenediamine,

Trimethylenediamine,

Tetramethylenediamine,

Hexamethylenediamine,

Decamethylenediamine,

Isophoronediamine,

m-Xylylenediamine,

Hydrazine, and

Water.

Among polyurethane resins containing as pendants residual groups of asiloxane compound which is obtained from materials such as thosementioned above, particularly preferred are those containing pendantresidual groups of a siloxane compound in a proportion of about 1.0-50wt.% per polyurethane resin molecule. If the proportion of the pendantresidual groups of the siloxane compound is smaller than about 1.0 wt.%,stick-free property, antiblocking property and the like cannot bebrought about to sufficient extents, thereby failing to achieve theobjects of the present invention. If the proportion exceeds 50 wt.% onthe other hand, the resulting polyurethane resin fails to havesufficient flexibility. It is hence not preferred to contain suchpendant groups in any proportion outside the above range.

The preferable molecular weight is 20,000-500,000 with 20,000-250,000being most preferred.

In this invention, it is also possible to react the siloxane compoundand polyisocyanate in a state rich in the isocyanate so that apolyurethane resin having at least one free isocyanate group is obtainedand is then used in combination with another film-forming resin as amodifier for the last-mentioned film-forming resin.

The polyurethane resins according to this invention, which containresidual groups of such siloxane compound as pendants, can be obtainedeasily by a production process known to date. These polyurethane resinsmay be produced without solvent or in an organic solvent. It is howeveradvantageous from the process standpoint to produce them in an organicsolvent, because the resultant solutions can be used for variousapplications as they are.

Preferred as such organic solvents are methyl ethyl ketone, methyln-propyl ketone, methyl isobutyl ketone, diethyl ketone, methyl formate,ethyl formate, propyl formate, methyl acetate, ethyl acetate, butylacetate, etc. It is also feasible to use acetone, cyclohexane,tetrahydrofuran, dioxane, methanol, ethanol, isopropyl alcohol, butanol,toluene, xylene, dimethylformamide, dimethylsulfoxide,perchloroethylene, trichloroethylene, methyl cellosolve, butylcellosolve, cellosolve acetate, etc.

In spite of the superb tack-free property and antiblocking property ofthe polyurethane resins according to this invention, they are soluble invarious organic solvents and can therefore form excellent flexiblefilms.

The polyurethane resins according to this invention therefore havesuperb solubility in various solvents and can provide films excellentnot only in flexibility but also in high-temperature tack-free property,antiblocking property and the like, so that they are useful for variousapplications, for example, as binders for the formation of magneticlayers of magnetic recording media such as magnetic tapes, back-coatingmaterials applied to the back sides of such magnetic recording media,binders for transfer films, binders for the formation of heat-resistantlayers effective for the prevention of sticking of a thermal head, basematerials for synthetic leather, fiber-coating agents, surface treatmentagents, binders for the formation of release layers such as partingpaper, binders for coating compositions and printing inks, etc.

As one example of applications of the polyurethane resins according tothis invention, formation of a heat-resistant layer of a thermalrecording material such as a thermal transfer film will next bedescribed.

The term "thermal recording material" means a recording material whichhas a transfer ink layer or sublimation transfer dye layer on one sideof a base sheet such as a polyester film and is heated in a pattern of amark from the back side by a thermal head to transfer the ink or dyeonto a receiving material thereby to reproduce the mark in a singlecolor or multiple colors. Upon transfer by means of the thermal head,the thermal head sticks on the back side so that various troubles aredeveloped.

The polyurethane resins according to this invention have excellentflexibility and in particular, superb tack-free property especially evenat elevated temperatures, and are therefore most suitable for such anapplication. Needless to say, the above application is merelyillustrative and the application of the polyurethane resins of thisinvention is by no means limited to the above example.

It is preferable for the formation of the heat-resistant layer to use acoating formulation which is formed by either dissolving or dispersingthe polyurethane resin of this invention either singly or in combinationwith one or more other resins in a medium such as that described above.The total concentration of film-forming resins in a coating formulationmay preferably be from about 10 wt.% to about 55 wt.%. The polyurethaneresin according to this invention, which contains as pendants residualgroups of the particular siloxane compound, may be used in a proportionof from about 1 to about 100 parts by weight per 100 parts by weight ofthese film-forming resins.

Various film-forming resins which have been known to date can be used incombination with the polyurethane resin of this invention. Suchconventional resins are all usable. Examples of such film-forming resinsinclude polyvinyl resins, polyvinylidene resins, vinyl chloride/vinylacetate/vinyl alcohol copolymer resins, alkyd resins, epoxy resins,acrylonitrile-butadiene resins, polyurethane resins, polyurea resins,nitrocellulose resins, polybutyrate resins, polyester resins, fluorineresins, melamine resins, urea resins, acrylic resins, polyamide resins,etc. Resins containing isocyanate-reactive groups in their structuresmay be used preferably, in particular, when the polyurethane resin ofthis invention contains free isocyanate groups as pendants. These resinsmay be used either singly or in combination and as either a solution ora dispersion in an organic solvent.

So long as the above-described components are contained as essentialcomponents, a coating formulation adapted to form a heat-resistant layermay contain auxiliary components in addition to the above components,for example, one or more desired additives such as pigment, extendingpigment, plasticizer, antistatic agent, surface active agent, lubricant,crosslinking agent, age resister, stabilizer, foaming agent and/ordefoaming agent.

Heat-resistant layers may each be formed by any method known to date,preferably, to a thickness of from about 0.1 μm to about 10 μm.

Any conventional base sheet can be used for the production of thethermal recording material according to this invention. For example,polyester films, polypropylene films, cellulose triacetate films,cellulose diacetate films, polycarbonate resin and the like can be usedas desired.

The thermal recording material can be produced successfully in a mannerknown per se in the art except for the use of such a polyurethane resinfor the formation of its heat-resistant layer. For example, the thermalrecording layer can be formed by a method known to date from a binderresin, a dye or pigment, an organic solvent, and various other additivesas needed.

For example, as the binder resin, it is possible to use a resin such asone of the aforementioned film-forming resins. Any one of the organicsolvents described above may be used as the organic solvent. Asexemplary pigments, may be used organic pigments such as azo,phthalocyanin, quinacridone and polycyclic pigments and inorganicpigments such as carbon black, iron oxides, chrome yellow and cadmiumsulfide. On the other hand, various dyes known to date, includingsublimate dyes and disperse dyes, can be used as dyes.

In thermal recording materials according to this invention, theirheat-resistant layers have high heat resistance and low heat tackinessof such degrees not available by any conventional techniques whileretaining various good properties, for example, solubility, flexibility,strength, and electrical, chemical and physical properties. In contrastto thermal recording materials according to the conventional techniques,thermal recording materials of this invention can therefore be used withextremely good stability without softening and/or sticking of theirheat-resistant layers under heat from a thermal head, so that theaforementioned drawbacks of the conventional techniques have beensolved.

Further, the pendant residual groups of the siloxane compound in eachpolyurethane resin according to this invention do not form the backboneof the film-forming resin but are bonded as side chains to thefilm-forming resin. Unlike conventional heat-resistant resins, thepolyurethane resins do not lower the excellent flexibility whichfilm-forming resins have inherently. The polyurethane resins useful forthe production of thermal recording materials in this invention are notlimited to their single use but can be used freely in combination withany film-forming resin or resins, thereby bringing about a furtheradvantage that thermal recording materials with a heat-resistant layermade of one or more of various film-forming resins and having high heatresistance and heat tackiness can be provided without pushing up theirproduction cost.

Since the heat-resistant layers of thermal recording materials of thisinvention are formed of a polyurethane resin such as that describedabove, they are free from the drawbacks of the conventional techniquesthat the heat-resistant component is allowed to breed out to thesurfaces of the heat-resistant layers along the passage of time therebyto smear and wear thermal heads.

The present invention will hereinafter be described more specifically bythe following Examples and Comparative Examples, in which alldesignations of "part" or "parts" and "%" mean part or parts by weightand wt.%.

EXAMPLE 1 ##STR3## wherein k is a value to give a molecular weight of5,200.

Dissolved in a mixed solvent composed of 200 parts of methyl ethylketone and 50 parts of dimethylformamide were 150 parts ofpolydimethylsiloxanepolyol (molecular weight: 5,200) having theabove-defined structure and 12 parts of 1,4-butanediol. While thoroughlystirring the contents at 60° C, a solution of 42 parts of hydrogenatedMDI in 128 parts of dimethylformamide was gradually added dropwise.After completion of the dropwise addition, they were reacted at 80° Cfor 5 hours to obtain a polyurethane resin of this invention in the formof a solution.

The solution had a solid content of 35% and a viscosity of 16,500 cps(at 25° C).

A film was obtained from the solution, whose strength (kg/cm2) at break,elongation (%) at break and softening point were 360, 350 and 92° Crespectively. Example 2 ##STR4## wherein k is a value to give amolecular weight of 5,200.

Dissolved in a mixed solvent composed of 250 formamide were 75 parts ofpolydimethylsiloxanepolyol (molecular weight: 5,200) having theabove-defined structure and 15 parts of 1,4-butanediol. While thoroughlystirring the contents at 60° C., a solution of 56 parts of hydrogenatedMDI in 110 parts of dimethylformamide was gradually added dropwise.After completion of the dropwise addition, they were reacted at 80° Cfor 5 hours to obtain a polyurethane resin of this invention in the formof a solution.

The solution had a solid content of 35% and a viscosity of 31,000 cps(at 25° C.).

A film was obtained from the solution, whose strength (kg/cm²) at break,elongation (%) at break and softening point were 460, 400 and 105° Crespectively.

EXAMPLE 3 ##STR5## wherein k is a value to give a molecular weight of4,500.

Dissolved in a mixed solvent composed of 150 parts of methyl ethylketone and 100 parts of dimethylformamide were 4 parts ofpolydimethylsiloxanepolyol (molecular weight: 4,500) having theabove-defined structure, 146 parts of polybutylene adipate (molecularweight: 2,000) and 10 parts of 1,4-butanediol. While thoroughly stirringthe contents at 60° C., a solution of 47 parts of hydrogenated MDI in134 parts of dimethylformamide was gradually added dropwise. Aftercompletion of the dropwise addition, they were reacted at 80° C for 6hours to obtain a polyurethane resin of this invention in the form of asolution.

The solution had a solid content of 35% and a viscosity of 45,000 cps(at 25° C).

A film was obtained from the solution, whose strength (kg/cm²) at break,elongation (%) at break and softening point were 450, 410 and 90° Crespectively.

EXAMPLE 4 ##STR6## wherein k is a value to give a molecular weight of4,800.

Dissolved in a mixed solvent composed of 150 parts of methyl ethylketone and 100 parts of dimethylformamide were 4 parts ofpolydimethylsiloxanepolyol (molecular weight: 4,800) having theabove-defined structure, 146 parts of polybutylene adipate (molecularweight: 2,000) and 10 parts of 1,4-butanediol. While thoroughly stirringthe contents at 60° C, a solution of 46 parts of hydrogenated MDI in 134parts of dimethylformamide was gradually added dropwise. Aftercompletion of the dropwise addition, they were reacted at 80° C. for 6hours to obtain a polyurethane resin of this invention in the form of asolution.

The solution had a solid content of 35% and a viscosity of 48,000 cps(at 25° C.).

A film was obtained from the solution, whose strength (kg/cm²). atbreak, elongation (%) at break and softening point were 420, 430 and 95°C respectively.

EXAMPLE 5

A polyurethane resin of this invention was obtained in the form of asolution in a similar manner as in Example 1 except that a siloxanecompound of the following structure was used in the same amount in placeof the siloxane compound in Example 1. ##STR7## wherein k is a value togive a molecular weight of 5,200.

The solution had a solid content of 35% and a viscosity of 48,000 cps(at 25° C.).

A film was obtained from the solution, whose strength (kg/cm²) at break,elongation (%) at break and softening point were 420, 430 and 95° C.respectively.

EXAMPLE 6

A polyurethane resin of this invention was obtained in the form of asolution in a similar manner as in Example 2 except that a siloxanecompound of the following structure was used in the same amount in placeof the siloxane compound in Example 2. ##STR8## wherein k is a value togive a molecular weight of 5,200.

The solution had a solid content of 35% and a viscosity of 48,000 cps(at 25° C.).

A film was obtained from the solution, whose strength (kg/cm²) at break,elongation (%) at break and softening point were 420, 430 and 95° C.respectively.

EXAMPLE 7

A polyurethane resin of this invention was obtained in the form of asolution in a similar manner as in Example 3 except that a siloxanecompound of the following structure was used in the same amount in placeof the siloxane compound in Example 3. ##STR9## wherein k is a value togive a molecular weight

of 5,200.

The solution had a solid content of 35% and a viscosity of 48,000 cps(at 25° C.).

A film was obtained from the solution, whose strength (kg/cm²) at break,elongation (%) at break and softening point were 420, 430 and 95° C.respectively.

EXAMPLE 8

A polyurethane resin of this invention was obtained in the form of asolution in a similar manner as in Example 4 except that a siloxanecompound of the following structure was used in the same amount in placeof the siloxane compound in Example 4. ##STR10## wherein k is a value togive a molecular weight of 5,200.

The solution had a solid content of 35% and a viscosity of 48,000 cps(at 25° C.).

A film was obtained from the solution, whose strength (kg/cm²) at break,elongation (%) at break and softening point were 420, 430 and 95° C.respectively.

COMPARATIVE EXAMPLE 1

Dissolved in a mixed solvent composed of 200 parts of methyl ethylketone and 50 parts of dimethylformamide formamide were 150 parts ofpolybutylene adipate (molecular weight: 2,000) and 15 parts of1,4-butanediol. While thoroughly stirring the contents at 60° C, asolution of 62 parts of hydrogenated MDI in 171 parts ofdimethylformamide was gradually added dropwise. After completion of thedropwise addition, they were reacted at 80° C. for 6 hours to obtain apolyurethane resin in the form of a solution.

The solution had a solid content of 35% and a viscosity of 32,000 cps(at 25° C.).

A film was obtained from the solution, whose strength (kg/cm²) at break,elongation (%) at break and softening point were 450, 480 and 102° C.respectively.

Incidentally, the softening point in each of the above Examples andComparative Example was determined by cutting the corresponding filminto a narrow strip, hanging in a Geer oven the strip with a weightapplied to a lower edge portion thereof to give a weight of 450 g/cm²,heating the oven at a rate of 2° C./min and then recording a temperatureat which the elongation of the strip increased abruptly or the strip wascut off.

COMPARATIVE EXAMPLE 2

Dissolved in 1,000 parts of toluene were 100 parts of a silicone resin"KS-841" (trade name; product of Shin-Etsu Chemical Co., Ltd.) and 1part of a catalyst "PL-7", whereby a coating formulation of the siliconeresin was prepared.

EXAMPLES 9-16 & Comparative Examples 4-6

Coating formulations of the following compositions were preparedseparately. They were separately applied to the back sides of 15-μmthick polyester films, on which a thermal recording layer had beenformed in advance, by a gravure coater to give a dry coat thickness of0.5 μm. The solvent was dried up in a dryer, so that heat-resistantlayers were formed. The thus-coated films were then cut into apredetermined width to produce thermal recording materials, which madeuse of the polyurethane resins according to this invention respectively,and comparative thermal recording materials.

EXAMPLE 9

    ______________________________________                                        Resin solution of Example 1                                                                           100 parts                                             Methyl ethyl ketone     100 parts                                             ______________________________________                                    

EXAMPLE 10

    ______________________________________                                        Resin solution of Example 2                                                                           100 parts                                             Methyl ethyl ketone     100 parts                                             ______________________________________                                    

EXAMPLE 11

    ______________________________________                                        Resin solution of Example 3                                                                           100 parts                                             Methyl ethyl ketone     100 parts                                             ______________________________________                                    

EXAMPLE 12

    ______________________________________                                        Resin solution of Example 4                                                                           100 parts                                             Methyl ethyl ketone     100 parts                                             ______________________________________                                    

EXAMPLE 13

    ______________________________________                                        Resin solution of Example 5                                                                           100 parts                                             Methyl ethyl ketone     100 parts                                             ______________________________________                                         EXAMPLE 14

    ______________________________________                                         Resin solution of Example 6                                                                          100 parts                                             Methyl ethyl ketone     100 parts                                             ______________________________________                                    

EXAMPLE 15

    ______________________________________                                        Resin solution of Example 7                                                                           100 parts                                             Methyl ethyl ketone     100 parts                                             ______________________________________                                    

EXAMPLE 16

    ______________________________________                                        Resin solution of Example 8                                                                           100 parts                                             Methyl ethyl ketone     100 parts                                             ______________________________________                                    

COMPARATIVE EXAMPLE 4

    ______________________________________                                         Resin solution of Comp. Ex. 1                                                                        100 parts                                             Methyl ethyl ketone     100 parts                                             ______________________________________                                    

COMPARATIVE EXAMPLE 5

    ______________________________________                                        Resin solution of Comp. Ex. 1                                                                         100 parts                                             Talc                     5 parts                                              Methyl ethyl ketone     120 parts                                             ______________________________________                                    

COMPARATIVE EXAMPLE 6

    ______________________________________                                         Resin solution of Comp. Ex. 2                                                                        100 parts                                             ______________________________________                                    

Evaluation

Characteristics of the thermal recording materials obtained respectivelyin Examples 9-16 and Comparative Examples 4-6 were as shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                 Coefficient                                                                             Sticking Head smear                                                 of friction                                                                             resistance                                                                             resistance                                        ______________________________________                                        Example 9  0.125       5        5                                             Example 10 0.173       5        5                                             Example 11 0.185       5        5                                             Example 12 0.203       5        5                                             Example 13 0.115       5        5                                             Example 14 0.162       5        5                                             Example 15 0.198       5        5                                             Example 16 0.205       5        5                                             Comp. Ex. 4                                                                              0.612       1        5                                             Comp. Ex. 5                                                                              0.352       3        2                                             Comp. Ex. 6                                                                              0.195       5        2                                             ______________________________________                                    

Each coefficient of friction in Table 1 is a measurement datum of thecoefficient of friction between an untreated polyethylene terephthalatesurface and the heat-resistant layer formed in the corresponding Exampleor Comparative Example.

Each sticking resistance was determined by visually ranking, in 5stages, the releasability of a thermal head from the correspondingthermal recording material when the thermal recording material was usedin an actual thermal recording test and the thermal head was pressedagainst the thermal recording material. Score of 5 was given to thoseshowed best sticking resistance.

Each hear smear resistance was determined by using the correspondingthermal recording material in an actual thermal recording test,observing the degree of smear of the thermal head and then ranking it infiber stages, score of 5 being given to those showed least smear.

From the above results, it is clearly understood that thermal recordingmaterials making use of a polyurethane resin according to this inventionare equipped with a heat-resistant layer having a low coefficient offriction and high sticking resistance and head smear resistance.

What is claimed is:
 1. A thermal recording material, comprising a basesheet, a thermal recording layer provided on one side of the base sheetand a heat-resistant layer applied on the other side of the base sheet,wherein the heat-resistant layer comprises a polyurethane resin whichcontains, as pendant groups, residual groups of a siloxane compoundhaving the formula (I) or (II) or both: ##STR11## wherein R is an alkylgroup, R' is a hydrogen atom or C₁ -C₅ alkyl group, k, l and m are1-250, 0-5 and 0-50, respectively, and n is ann integer of 1-3 in theformula (I) and an integer of 2-3 in the formula (II), said siloxanepolyol component being present in an amount sufficient to provide saidpolyurethane resin with tack-free and anti-blocking properties, whileretaining resin flexibility.
 2. The thermal recording material asclaimed in claim 1, wherein said polyol component further contains, incombination with said siloxane compounds, a polyol containing hydroxylgroups as terminal groups and having a molecular weight in the range of300-4000.
 3. The thermal recording material as claimed in claim 2,wherein said polyol is selected from the group consisting ofpolyethylene adipate, polyethylene propylene adipate, polyethylenebutylene adipate, polydiethylene adipate, polybutylene adipate,polyethylene succinate, polybutylene succinate, polyethylene sebacate,polybutylene sebacate, polytetramethylene ether glycol,poly-ε-caprolactone diol, polyhexamethylene adipate, and polypropyleneglycol, and any of the above also containing a proportion ofpolyoxyethylene chains.
 4. The thermal recording material as claimed inclaim 1, wherein said polyisocyanate component is selected from thegroup consisting of 4,4'-diphenylmethane diisocyanate, hydrogenated4,4'-diphenylmethane diisocyanate, isophorone diisocyanate,1,3-xylylene, diisocyanate, 2,6-tolylene diisocyanate, 2,4-tolylenediisocyanate, 2,6-tolylene diisocyanate, 1,5-naphthalene diisocyanate,m-phenylene diisocyanate and p-phenylene diisocyanate.
 5. The thermalrecording material as claimed in claim 1, wherein said siloxane compoundis used in an amount of about 1.0-50 weight %.
 6. The thermal recordingmaterial as claimed in claim 1, having a molecular weight of from20,000-500,0000.
 7. The thermal recording material as claimed in claim6, a molecular weight of from 20,000-250,000.